Description

Annotations are some kind of objects which are intended to interact with users.
There is a large number of standard annotation types provided by PDF:
* Text - "sticky note" attached to specified point in document.
* Link - link to another page or URI.
* Free Text - display text on a page (unlike ordinary text annotation which displays text in a pop-up window).
* Line - line, displayed on a page, opening displays a pop-up window containing the text of the associated note.
* Square and Circle - display, respectively, a rectangle or an ellipse on the page. When opened, they display a pop-up window containing the text of the associated note.
* Markup - appear as highlights, underlines, strikeouts or jagged ("squiggly") underlines in the text of a document. When opened, they display a pop-up window containing the text of the associated note.
* Rubber Stump - displays text or graphics intended to look as if they were stamped on the page with a rubber stamp. When opened, it displays a pop-up window containing the text of the associated note.
* Ink - represents a freehand "scribble" composed of one or more disjoint paths. When opened, it displays a pop-up window containing the text of the associated note.
* Pop-up - displays text in a pop-up window for entry and editing. Usually appears as child annotation.
* File attachment - reference to embedded file. Allows to store this file in the file system.
* Sound annotation - plays sound when activated.
* Movie - play movie when activated.
* Widget - an interactive form related object.
* Printer's Mark - "a graphic symbol, such as a registration target, color bar, or cut mark, added to a page to assist production personnel in identifying components of a multiple-plate job and maintaining consistent output during production".
* Trap Network - trap networks, associated with a page.

Text, Link, Line, Square and Circle, Murkup, and File Attachment are most usable and easy to implement.